Input devices are widely used in a variety of electronic systems. Input devices include touch sensor devices (also commonly called touchpads or proximity sensor devices) and fingerprint sensor devices. Touch sensor devices typically include a sensing region in which the touch sensor device determines the presence, location and/or motion of one or more input objects, typically for purposes of allowing a user to provide user input to interact with the electronic system. Fingerprint sensor devices also typically include a sensing region in which the fingerprint sensor device determines presence, location, motion, and/or features of a fingerprint or partial fingerprint, typically for purposes relating to user authentication or identification of a user. The sensing region of a touch sensor device or a fingerprint sensor device may be demarked by a surface.
Touch sensor devices and fingerprint sensor devices may thus be used to provide interfaces for an electronic system. Examples of touch sensor devices and fingerprint sensor devices include opaque touchpads and fingerprint readers integrated in or peripheral to laptop or desktop computers. Other examples of touch sensor devices and fingerprint sensor devices include touch screens integrated in mobile devices such as smartphones and tablets.
Touch sensor devices and fingerprint sensor devices, as well as other types of devices, may include “always on” idle states in which the device consumes low power relative to a higher power state in which the device is performing some operation. An example of an “always on” idle state is a wake-on-event (WOE) state. An example of a relatively higher power state is a touch sensor device or a fingerprint sensor device performing imaging. In the “always on” idle state, power is supplied to the device, and if there is a power event such as a glitch or malfunction in which the voltage provided by the power supply falls too low, problems such as memory corruption may occur.
Conventionally, bandgap voltage comparators may be used to detect a low voltage event. A status signal provided by the bandgap voltage comparator can indicate whether or not a power supply is in an operational state or whether it is too low. For example, a high value of the status signal can correspond to a power status “good” state, and a low value of the status signal can correspond to a power status “bad” state. The bandgap voltage comparator may, for example, be part of a power-on-reset (POR) circuit. When there is a transition from the operational state to a low power state which may cause a component of the device to malfunction, the output of the bandgap voltage comparator can be used to respond to the situation and avoid the malfunction. For example, the output of the bandgap voltage comparator indicating a “bad” power status can trigger a response to avoid a problem such as a chip malfunction. In another situation, when there is a transition from a low power state to an operational state, the output of the bandgap voltage comparator indicating a “good” power status can be used to safely enable one or more components of the device.
However, bandgap voltage comparators have the disadvantage of requiring a relatively large bias current, which can result in a significant amount of cumulative power consumption during an “always on” idle state.